In the cylindrical workpieces on lathes, collets are frequently used to chuck the workpieces. This is even more so in the case of the working of bar stock on semiautomatic or fully automatic single-spindle or double-spindle lathes. In the case of a large number of these lathes, the collets are closed by means of a tightening device. In the case of collets produced in one piece, it is possible to distinguish between a closer part and a draw-in tube part. On its inside, the closer part has clamping surfaces for the workpieces and on its outside has an external cone which widens towards the free end face. The closer part is divided into individual clamping jaws by longitudinal slots arranged distributed over the periphery. These longitudinal slots are through slots in the radial direction and extend in the axial direction from the free end face of the closer part to a longitudinal portion which remains unslotted and forms the actual draw-in tube part. At the end of this draw-in tube part there is a securing thread for securing the collet on the tightening device of the lathe. The central longitudinal portion situated between the actual closer part and the actual draw-in tube part is divided by the longitudinal slots into individual tongues by means of which the clamping jaws adjoining at the forward end can spring inwards and outwards in the radial direction relative to the draw-in tube part. In the lathe, the external cone of the closer part rests against a matching internal cone of the work spindle of the lathe.
During the closing or tightening movement, the clamping jaws situated at the forward end of the tongues move on an arcuate movement path and in the process execute a swivelling movement. If a workpiece does not have exactly the nominal diameter but is overdimensioned or underdimensioned, it is gripped and chucked only by the rearward or by the forward end of the clamping surfaces, i.e. only over a relatively small longitudinal portion. The bending moment produced as a result at the clamping jaws is generally not capable of deforming the tongue elastically to such an extent that the clamping surfaces of the clamping jaws come to rest over a larger longitudinal portion or even over their entire length against the workpiece. As a result, there is the risk that the workpiece will avoid the cutting forces and execute wobbling movements, leading to a poor machining result. To reduce this risk, the collets of a collet set must be very narrowly stepped, resulting in a large number of collets and correspondingly high costs for a collet set. In addition, these circumstances make it necessary to pay attention to small diameter tolerances in the workpieces and, for example, in the working of bar stock to do without the cheaper rolling material and instead use more expensive drawn or bright-turned bars.
A certain remedy, specifically with respect to the tilting of the clamping jaws, is achieved in a collet (French Patent No. 1,371,605) in which the closer part and the draw-in tube part are separate from one another and are coupled to one another by means of a coupling device. For this purpose, a circumferential groove is present either in the outer circumferential surface of the draw-in tube part or in the inner wall surface of the closer part and an inward- or outward-protruding collar matched to this groove is present on the counterpart. Because of the longitudinal slots, this collar is divided at the clamping jaws of the closer part into individual arcuate claws. At the draw-in tube part, the axial tensile force for chucking a workpiece is introduced via the neck, situated in the same radial plane as the groove, into the adjoining collar-shaped longitudinal portion, transmitted by the end face of the latter, which end face faces the claws, to said claws and transmitted in turn by the claws into the clamping jaws via the neck, which is situated in the same radial plane as the collar on the draw-in tube part. Over the entire chucking capacity of the collet, i.e. even in the case of a clamping diameter which is smaller relative to its nominal diameter, a certain safety clearance must be present between the cylindrical inside surface of the claws and the groove base on the draw-in tube part in order to be able to bridge production tolerances, inclusions of foreign bodies, rounding radii in the groove base with a view to eliminating the notch effect, and the like, without bottoming. This safety clearance and the difference in the radius of the desired chucking capacity must be subtracted from the radial dimension of the annular cross-sectional area between the penetration orifice in the neck of the draw-in tube part and the circumferential surface at the clamping jaws in the radial plane of the collar on the draw-in tube part. In the case of the two neck surfaces, account must be taken of the fact that, although the penetration depth of the case-hardening necessary for the contact surfaces provides a high wear resistance, it does not provide high tensile strength. In addition, the stress increase as a result of the notch effect due to the abrupt changes in cross-section at the transition from the neck to the adjoining collar and to the claws must be taken into account. The available radius region must therefore be divided in such a way that only a relatively small radius region is available for the contact surfaces and only a very small radial relative movement and hence only a very small chucking capacity can be permitted. In addition, for reasons of strength, the axial loading of the coupling device must be limited.
In another known collet disclosed in U.S. Pat. No. 4,214,766 (German Patent No. 2,732,677), these difficulties were reduced by the fact that axial extensions, arranged distributed in the circumferential direction and engaging claw-fashion in one another in the axial direction, and recesses situated between said extensions are present at the mutually facing ends of the closer part and of the draw-in tube part, that a circumferential groove is in each case present on the closer part and on the draw-in tube part in the region of the axial extensions and that a coupling ring is inserted into said groove. This coupling ring is divided at two diametrically opposite locations on the circumference. At in each case one of the two adjacent ends, the coupling ring is swivel-mounted on the draw-in tube part by means of a swivel joint having a swivelling axis oriented parallel to the longitudinal axis of the collet. At the other end of the two coupling ring halves is a position-securing device for the coupling ring halves which is formed principally by a locking pin which, in the clamping position of the coupling ring halves is pushed into mutually aligned bores at the end of each coupling ring half and at the closer part. In this coupling device, the radius difference remaining after subtraction of only one safety clearance and of the radial chucking capacity has to be divided over only two force-transmission points, namely over the end-face contact surface between the coupling ring and the groove in the axial extensions of the collet parts and over the neck surface of these extensions. However, these advantages are achieved at the expense of the fact that, to exchange the closer part for another clamping diameter, the entire collet must be pushed at least far enough out of the work spindle of the lathe for the position-securing device of the coupling ring halves to be freely accessible, and to enable the two coupling ring halves, after the release of the position-securing device, to be swivelled out of the circumferential groove of the extensions engaging in one another claw-fashion. In the case of an exchange of the clamping diameter, this requires a relatively large displacement path for the collet and furthermore requires relatively involved handling of the parts of the coupling device between the draw-in tube part and the closer part. In addition, for this purpose there must be sufficient space for the movement of the coupling ring halves in the vicinity of the work spindle, both in the axial direction and in the radial direction, but this space is frequently lacking especially in the case of multi-spindle automatic lathes.